Sewage sludge is a byproduct of wastewater treatment, either primary or secondary. The disposal of sewage sludge from wastewater treatment plants is a growing environmental problem throughout the world, especially in the United States, Europe and Japan. In many areas of the world the dumping of sludge into the ocean has recently been banned, such as, for example, western Europe and the United States. Generally, there is a pressing need for economical and environmentally safe methods for disposing of sewage sludge.
Present methods for disposal of sewage sludge include landfilling, land application, composting and pelletization for making fertilizers, incineration, and wet oxidation. Land application, composting, and pelletization, which attempt to utilize the sludge as a fertilizer, have quality requirements on such things as the heavy metals content in the sludge. If these limitations are exceeded, disposal options are normally limited to landfilling, incineration, and wet oxidation. In highly industrial areas such as the northeastern United States, southeastern Canada, western Europe and Japan, a compounding of factors further aggravates the sludge disposal problem. In these areas sludge volumes tend to be the highest, sludge quality is often poor (high heavy metals), and landfill space is scarce.
Another waste with disposal challenges similar to sewage sludge is municipal solid waste (MSW). Although the physical properties of sewage sludge and MSW are very different, their elemental compositions on a dry basis are very similar, with primary components of carbon, hydrogen, oxygen, and ash. The major difference between sewage sludge and MSW is in their moisture contents. MSW generally contains about 20 to 30 wt. % moisture. Raw sewage sludge from wastewater treatment is essentially a liquid slurry, and normally contains only 2 to 4 wt. % solids in water. To reduce the weight and volume for disposal, wastewater treatment plants often dewater the sludge to between 15 and 30 wt. % solids by mechanical means. Mechanical dewatering causes a drastic change in the physical properties of sewage sludge, converting it from a liquid slurry to a moist solid, with a consistency similar to wet dirt or mud.
The primary method of MSW disposal in the United States is landfilling. However, incineration of MSW does account for more than 15% of MSW disposal, and the MSW incinerators tend to be located in the more populated urban areas. Coincineration of sewage sludge with municipal solid waste is an alternative to sludge incineration that eliminates the need to build a stand alone incinerator. While this is advantageous, the high moisture content and low net heating value of dewatered sewage sludge have a significant detrimental effect on the combustion conditions in the MSW incinerator. Specifically, the addition of sewage sludge results in decreased combustion temperature and increased flue gas flow rate relative to the combustion of MSW alone. In addition, the physical properties of dewatered sewage sludge make it difficult to dry and combust in a typical moving grate MSW incinerator. The sludge often exits the incinerator dried and seared on the outside surface, but still moist and unburned on the inside.
Accordingly, a need exists for a process to coincinerate a significant quantity of dewatered sewage sludge, or other high moisture sludge material, in an MSW incinerator, without changing the combustion conditions of the incinerator from operation without sludge, and while maintaining a low unburned carbon content in the ash.
It has previously been suggested to coincinerate sewage sludge with MSW and attempts have been made to coincinerate raw sludge, dewatered sludge and thermally dried sludge. Many of such suggested techniques have proven to be impracticable or less than what might be desired.
Raw sewage sludge from a wastewater treatment plant normally contains 2 to 4 wt. % solids. Additional thickening equipment can increase the solids content possibly as high as 8 wt. % solids. The thickened sludge, however, still has the physical properties of a flowable liquid slurry.
Utilization of spray techniques to inject thickened sludge (typically 5-8 wt. % solids) directly into the combustion chamber of a solid waste incinerator was investigated in the 1970s. Limited trials with 3.5 wt. % solid sludge and a dual fluid spray nozzle were conducted to atomize sludge for coincineration into a full-scale refuse incinerator located in Havant, United Kingdom. The sludge to MSW ratio was limited in the tests to 3 wt. %. Full scale installation of the sludge atomization feed system at Havant was abandoned.
U.S. Pat. No. 3,322,079 describes a method and apparatus for incinerating sewage sludge and municipal waste. In this patent, the apparatus described is comprised of an incinerator housing with a traveling or conveying grate for solid waste fuel. A device, such as a centrifugal atomizer, is used to feed sludge into the combustion gases above the bed of burning solid waste. The atomizer is installed in a drying chamber, an extension of the combustion chamber, that requires a hot gas recirculating blower to contribute to the centrifugal dispersion of sludge particles, and also to heat the sludge to ignition temperature. The patent states that it is desirable to operate at a sludge feed rate, MSW feed rate, and combustion air flow such that the exhaust gas from the combustion chamber is maintained at 1400.degree. F. Without oxygen enrichment, however, the sludge/MSW ratio must be limited to impracticably low levels.
As explained above, dewatered sludge is obtained by mechanical processing (such as a belt press, filter press, or centrifuge), and normally contains between 15 and 30 wt. % solids. Attempts to coincinerate dewatered sludge with MSW have been limited to very low ratios of sludge relative to MSW. Dewatered sludge feed methods have included premixing the sludge and MSW in the MSW storage pit, dropping the sludge into the MSW feed chute, and spreading the sludge directly on the burning bed of trash via elaborate mechanical means. These operations are plagued by three major problems:
1. Unburned sludge material remaining in the bottom ash, caused by failing adequately to mix the sludge or break it into small enough particles; PA1 2. Low sludge capacity relative to MSW (2-3 wt. % on a dry sludge basis) due to decreasing combustion temperature and increasing flue gas flowrate with dewatered sludge addition. PA1 3. Unacceptably high concentrations of carbon monoxide in the flue gas due to lower temperatures in the combustion zone.
The present invention solves the above problems to enable the successful coincineration of mechanically dewatered sewage sludge, containing from 15 to 30 wt. % solids, with MSW in high ratios, for example, 10 wt. %, or more (e.g., 15% or even greater), sludge (dry basis) to MSW. In theory the upper limits on sludge flowrate and solids content are not limited by the sludge atomization nozzle or oxygen-enriched coincineration process, but by the sludge feed system. A single dual-fluid nozzle should be able to feed dewatered sludges as high as 25 to 30 wt. % solids. Since multiple nozzles could be used, the upper limit on sludge to MSW will be set by other constraints that will vary with the incinerator design, such as flue gas dew point (which increases due the moisture in the sludge), or oxygen compatibility of the combustion air ducts which will limit the level of oxygen enrichment.
To reduce the moisture content below that which mechanical dewatering can achieve requires thermal drying of the sludge. This can be accomplished in either direct or indirect contact equipment, such as, for example, heat recovery from the combustion flue gas. Thermally dried sludge can contain from 60 to 90+ wt. % solids. Thermal drying also causes significant physical changes to the sludge, producing a very dry, powdery product that ignites and burns easily.
To coincinerate successfully a significant fraction of sewage sludge relative to MSW using previously suggested technologies requires the sludge to be thermally dried.
Since dried sewage sludge has a net lower heating value equal to or greater than MSW, heat is released in the combustion of the sewage sludge solids. Further, the combustion of additional waste material does increase flue gas and combustion air flow rates relative to operation with MSW alone, which must be accounted for in a new design and decreases MSW capacity in a retrofit.
Another previous suggestion includes U.S. Pat. No. 4,630,555, which describes a cyclic process for incinerating waste which uses pure oxygen along with liquid (water) injection for temperature control. The waste is charged to the furnace in batches, and the oxygen and liquid are injected through a nozzle. The flowrates of oxygen and the liquid change throughout each cycle to first initiate combustion and then control combustion temperature.
Still another suggestion is described in U.S. Pat. No. 4,056,068 relating to a process to spray water into the flue gas of a waste incinerator above the secondary air nozzles to accelerate catalytically CO oxidation of unburned material in the combustion gases.
Yet another suggestion is shown in U.S. Pat. No. 5,147,563 which reveals a process and apparatus for aerobic treatment of sewage sludge. In this process an oxygen-rich or ozone-rich gas is injected with sludge into a dispersing assembly to atomize and oxygenate the sludge.
More recently, U.S. Pat. No. 5,052,310 entitled "Solid Waste-to-Steam Incinerator Capacity Enhancement by Combined Oxygen Enrichment and Liquid Quench," assigned to Air Products and Chemicals, Inc., describes a process to dispose of a high moisture containing waste (such as sewage sludge) in a MSW incinerator by enriching the combustion air with oxygen. This patented process employs the synergistic combination of oxygen enrichment with the high moisture content in the waste, which enables the coincineration of large quantities of the wet waste in an existing MSW incinerator without changing the combustion temperature, excess oxygen in the flue gas, and flue gas flowrate. This patented process also is sufficiently broad so as to encompass the utilization of just water (i.e., 100% moisture, 0% solids).
The present invention addresses issues such as the means or manner for introducing sewage sludge, or other similar high moisture material having a BTU or fuel value, into an MSW incinerator to ensure that it dries and burns completely within the limitations of that system (grate design, furnace residence time, temperature, etc.) and which combustion air streams should be enriched to optimize the combustion process on the basis of carbon burnout in the ash and gaseous emissions of CO and NO.sub.x.